Vehicle mounted concrete mixing drum and method of manufacture thereof

ABSTRACT

A method of manufacture of a vehicle mounted rotary concrete mixing drum of the type having an opening at one end for receiving and/or discharge of concrete therefrom and at the other end, means for engaging a drive assembly so as to rotate the drum for mixing or discharging concrete. The drum is manufactured from at least one mould using at least one plastics material and further includes integrally attached vanes which outstand from the internal surface of the drum forming an archimedian spiral disposed such that when the drum is rotated in a first direction the concrete contents are mixed and when the drum is rotated in a second direction the contents are discharged from the drum; wherein, the method comprises the steps of; a) preparing a first generally helical inner mold part containing a surface extending between first and second helical edges; b) mounting the first helical inner mould part on a support c) enclosing the inner helical mold assembly within an outer mould formed by at least one outer mold part; d) fitting a second mating inner helical mold part to the first inner mould part to form an inner mold assembly; e) injecting a polyurethane elastomer into a cavity defined by said inner mold assembly and the outer mould assembly to form an inner wall element comprising one half of an interior wall of the mixer and one helical blade; f) allowing said polyurethane to cure; g) removing said at least one outer mold parts to expose said inner wall element; h) removing said inner wall element one of said inner molds;

BACKGROUND

The present invention relates to concrete mixing apparatuses and moreparticularly relates to a vehicle mounted plastics drum for mixingconcrete and a method manufacture thereof

PRIOR ART

The building industry makes widespread use of concrete mixing trucks fortransportation of ready mixed concrete to sites for concrete pours.These trucks typically comprise a large mixing assembly including amixer drum mounted to the vehicle and which is connected to a mixerdrive for mixing concrete contents during transportation and fordischarge of the contents on site. The drive system comprises a gear boxwhich takes power from the vehicle motor and which applies a mixingtorque to the drum imparting axial rotation to the drum with the torquebeing adjustable depending upon the operating requirements. The abovegeneral arrangement is described in U.S. Pat. No. 4,585,356 whichdiscloses a concrete mixer truck having a mixer drum adapted to berotated by the traction motor of the vehicle through an auxiliarytransmission of the traction motor transmission.

According to the known vehicle mounted mixing assemblies, the mixingdrum is typically of heavy duty steel construction and is disposed atapproximately 10 to 15 degrees from horizontal. The drum is fitted withinternal vanes or mixing blades defining an archimedian spiral so thatas the drum rotates in a first direction the concrete held therein ismixed and as the drum is rotated in the opposite direction, the concreteis discharged from the drum via an elevated discharge orifice under thereverse action of the internal spiral vanes. The drum is disposed suchthat the drive end is lowest and the discharge end is highest relativeto a generally horizontal plane of the vehicle.

While steel drums have been in use for many years, they suffer from anumber of attendant disadvantages relating to their cost of manufactureand replacement, working life, wear characteristics, weight and volume.

Steel drums are expensive to manufacture due to their labor intensiveconstruction which involves rolling steel sheets into conical portionsand cylinders which once fabricated are then welded to form the finishedtank. The archimedian spirals formed from flat sheets are then weldedinto position inside the drum. As concrete is a highly abrasivematerial, internal surfaces of steel drums are subject to significantwear abrasion. This occurs particularly on the surfaces which take slumpimpact, sliding friction and shear load leading to eventual wearing outof the drum.

Typically, a steel drum used every day might last three to five years,thereafter requiring replacement at significant cost. The abrasion ofinternal surfaces is increased where there are changes of slope in thedrum walls usually where the segments of the drum are joined.

The mixing blades are welded to the internal surface of the drum causingsharp angled recesses in which concrete can gather and eventually buildup degrading the internal surface and providing a catchment for furtherunwanted build up of concrete. By its nature, a steel surface isrelatively smooth and whilst this may be desirable for the purpose ofpreventing concrete build up on the walls of the drum, the interfacebetween the concrete and steel wall is an area of abrasion rather thanconcrete mixing.

Ideally, mixing of concrete should take place throughout the whole mix,but in the steel drums, optimum mixing does not take place at theboundary layer and in crevices in which concrete may collect. In fact,due to the nature of the frictional interface between the steel surfaceand concrete boundary layer, laminar flow occurs resulting in little orno mixing at the boundary layer. The reason for this is that theaggregate in the concrete slides and abrades (with reduced or no mixing)rather than rotates to facilitate mixing. Thus there are ‘dead’ spots inthe mix where no mixing takes place and where there is an increasedpotential for unwanted collection of concrete. In addition to the aboveproblems associated with the use of steel mixing drums, there are costand weight factors which add to inherent inefficiencies in use of steeldrums. Due to the dead weight of the steel drum, its volume must berestricted so the combination of the dead weight and concrete weightmust be maintained within the maximum allowable loading limits for thevehicle to which the drum is attached.

An alternative to the known steel drums was proposed in PCTInternational patent application PCT/AU00/01226 to Rodgers and Khouri.That application teaches the possibility of using a lightweight materialsuch as plastics for construction of a concrete mixing drum as asubstitute for steel whilst recognizing that there were numerousstructural and manufacturing difficulties to be overcome in making thetransition to plastics not the least of which was the production of adrum which could withstand the high static and dynamic loadings to whichtruck mounted mixing drums are subject to in normal operation. If theweight of the drum could be reduced without compromising and possiblyincreasing drum volume, the weight reduction could be taken up withadditional concrete thereby increasing the pay load.

There are variety of concrete mixing drum arrangements disclosed in theprior art none of which as far as the inventor is aware anticipate themethod of manufacture of a plastics drum to be described herein.

U.S. Pat. No. 4,491,415 discloses a lightweight, pear shaped rotarymixing device open at one end and having an axially elongated socket atthe large end. The drum is rotatably supported on a unitary base havinga transversely extended forward end and an upwardly and angularlyextending rear end providing a bearing portion detachably engagable withthe socket to rotatably support the drum at an inclination of about 35degrees. The drum has a plurality of axially extending radial fins forlifting contents in rotation thereof and is preferably fashioned frommolded plastics material either as a unitary body or as a plurality ofintermitting parts. The drum described in this patent is for light dutyoperation and does not have the structural and materials characteristicsnecessary for heavy duty concrete mixing operations.

U.S. Pat. No. 5,118,198 discloses a cement mixing apparatus with acradle support assembly and including a polyethelyne cement mixing drumheld and supported by a cradle arm assembly formed of cradle basesupport braces and upright cradle arms which interfit into cradle armrecesses which are preformed with the polyethylene drum. A bull geardrives the polyethylene drum. The drum disclosed in this patent isintended for light duty cement operations and does not address thestructural and manufacturing requirements for heavy duty operations.U.S. Pat. No. 5,492,401 discloses a concrete mixer with a mixing drumconsisting of high density crosslinked polyethylene material. The drumincludes a bottom supported buy a conventional rigid metal pan securedto the external surface thereof to rigidify the plastic drum and extendthe life expectancy of the plastic drum by enabling the concrete mixerto be used to complete a mixing job at a job site even though movementof the concrete mix within the drum during repetitive mixing cycles mayultimately wear a hole through the bottom of the plastic drum. Paddleassemblies are positioned interiorly of the drum and oriented tomaintain minimum splashing during the mixing operation. Not only is thedrum disclosed in this patent unsuitable for heavy duty vehicle mountedoperation the patent in fact teaches a means to accommodate a wearfailure on site whereby a hole could be worn through the wall of thedrum.

The prior art teaches use of plastics drums for small cement mixingoperations. However there are inherent difficulties in manufacturingplastic drums to an acceptable standard of strength and reliability.Plastics drums require use of materials which for a drum profile by useof a mould. As the discharge opening to a drum is narrower than theremainder of the drum, it is not possible to remove a mould from aninner surface unless the drum is made in sections which can be joined toform the drum structure. A number of methods of manufacture of plasticsheavy duty mixing drums have been proposed in PCT applicationPCT/AU00/01226 which discloses a heavy duty rotary concrete mixing drumfor attachment to a vehicle which is characterized in that the drum ismanufactured from at least one mould and from at least one plasticsmaterial and wherein the drum includes an inner surface having aproperty which promotes mixing of the concrete at the boundary layerbetween the concrete and said inner surface and reduces wear.

A number of different methods were proposed in that application for themanufacture of a drum of the type disclosed.

INVENTION

The present invention seeks to provide an alternative method ofconstruction of a heavy duty vehicle mounted rotating cement or concretemixing drum fabricated from plastics materials. The drum produced by themethod of the invention described herein overcomes the aforesaiddisadvantages of the prior art and maintains efficient concrete mixingcharacteristics. According to the invention there is provided a methodof construction of a plastics concrete mixing drum wherein the methodincludes the use of inner and outer molds each made up from separatemould parts which are divided along two helical lines mid way betweenthe mixing blades thereby allow formation of a drum interior from twoidentical molds.

In one broad form the present invention comprises:

a method of manufacture of a vehicle mounted rotary concrete mixing drumof the type having an opening at one end for receiving and/or dischargeof concrete therefrom and at the other end, means for engaging a driveassembly so as to rotate the drum for mixing or discharging concrete;wherein, the drum is manufactured from at least one mould using at leastone plastics material; wherein the drum further includes integrallyattached vanes which outstand from the internal surface of the drumforming an archimedian spiral disposed such that when the drum isrotated in a first direction the concrete contents are mixed and whenthe drum is rotated in a second direction the contents are dischargedfrom said drum; wherein, the method comprises the steps of;

a) preparing a first generally helical inner mold part containing asurface extending between first and second helical edges;

b) mounting the first helical inner mould part on a support

c) enclosing the inner helical mold assembly within an outer mouldformed by at least one outer mold part;

d) fitting a second mating inner helical mold part to the first innermould part to form an inner mold assembly;

e) injecting a polyurethane elastomer into a cavity defined by saidinner mold assembly and the outer mould assembly to form an inner wallelement comprising one half of an interior wall of the mixer and onehelical blade;

f) allowing said polyurethane to cure;

g) removing said at least one outer mold parts to expose said inner wallelement;

h) removing said inner wall element one of said inner molds;

The method preferably comprises the additional step of placing areinforcing member in a recess formed in said inner mold part. Stepsa)-h) are repeated thereby providing a second helical inner wallelement. The first and second helical inner wall elements arecomplimentary and combine to form an inner wall surface of the mixingdrum. The first and second helical inner wall elements are thenpreferably placed into a jig where opposing edges of said elements areheld adjacent; the wall elements defining an inner cavity of said drum.Opposing edges of the elements are sealed to complete the inner wall ofthe drum. The inner wall is removed from the jig and placed the innerwall on a mandrel such that the mandrel is disposed in the inner cavity.The inner wall is placed on the mandrel via an open end of the innerwall whereupon structural layers of glass fibre reinforced plastic areapplied to the polyurethane inner layer. Preferably, the internalsurface of the drum includes an elastomer which will allow mixing of thecontents of the concrete at a concrete boundary layer;

The reinforcing member is preferably a continuous rope inserted in arecess in a formation which will form helical blades.

The method includes the use of inner and outer molds each made up fromseparate mould parts which are divided along two helical lines therebyallowing formation of a drum interior wall from two complementary mouldparts.

In another broad form the present invention comprises: a method ofmanufacture of a vehicle mounted rotary concrete mixing drum of the typehaving an opening at one end for receiving and discharge of concretetherefrom and at the other end means for engaging a drive assembly so asto rotate the drum for mixing or discharging concrete; wherein, the drumis manufactured from at least one inner mould and at least one opposingouter mould; wherein the drum includes integrally attached vanes whichoutstand from the internal surface of the drum forming an archimedianspiral disposed such that when the drum is rotated in a first directionthe concrete contents are mixed and when the drum is rotated in a seconddirection the contents are discharges from said drum; and wherein theinternal surface of the drum is formed or lined with an elastomer whichcauses mixing of the contents of the concrete at the concrete boundarylayer; wherein the method comprises the steps of;

a) preparing a first inner helical mold containing a surfaceintermediate side edges of the mould;

b) placing a reinforcing rod in a recess in said inner mold;

c) enclosing the inner helical mold assembly within at least one outermold part;

d) sealing a joint between said inner mold part and said at least oneouter mold part;

e) injecting a polyurethane elastomer into a cavity defined by saidinner mold and said at least one outer mold part to form an innerhelical wall element comprising one half of an interior of the mixer andone helical blade;

g) allowing said polyurethane to cure;

h) removing said at least one outer mold parts;

i) removing one of said inner molds;

j) removing the interior polyurethane inner helical wall element moldingfrom the remainder of the inner mold assembly;

k) repeating steps a)-j) to form a second inner wall element.

Preferably when a first and a second inner wall element are placed in ajig jointing surfaces are held adjacent to enable sealing the joinformed by the jointing surfaces.

According to a preferred embodiment, a mandrel is inserted into an opendischarge end of a drum interior following which structural layers ofglass fibre reinforced plastics are wound about an outer surface of theinner wall.

In another broad form the present invention comprises:

a method of construction of a plastics mixing drum comprising the stepsof:

a) preparing a first inner mold containing a surface extending from ajoint line midway between two helical blades to a mid line mould jointline at an inner edge of said blades;

b) placing a reinforcing rod in a recess in said inner mold;

c) fitting a second mating inner helical mold to form an inner moldassembly;

d) enclosing the inner helical mold assembly within at least one outermold part;

e) sealing a joint between said inner mold assembly and said at leastone outer mold part;

injecting a polyurethane elastomer into a cavity defined by said innermold assembly and said at least one outer mold part to form one half ofthe interior of the mixer and one of the helical blades;

f) allowing said polyurethane to cure;

g) removing said at least one outer mold parts;

h) removing one of said inner molds;

i) removing the interior polyurethane molding from the remainder of theinner mold assembly;

j) placing said two helical blade and interior moldings in a jig wherethe jointing surfaces are held adjacent;

k) inserting a mandrel into an open discharge end of the drum;

l) applying structural layers of glass fibre reinforced plastic thepolyurethane layer.

Preferably the reinforcing is fitted using spacers which centralize thereinforcing rod or rope in its recess. Preferably the first and secondinner helical mold elements are jointed with a sealing compound orgaskets along an inner edge. According to one embodiment, the outer moldis formed in three separate mold parts.

Preferably, a joint between the two inner helical elements forming ahelical blade is made with a polyurethane elastomer compound.

In another broad form the present invention comprises:

a method of manufacture of a vehicle mounted concrete mixing drumcomprising the steps of;

a) taking a helical inner mould part and mounting the mould on asupport;

b) placing at least one external mould in opposing relationship to saidinner mould;

c) injecting a flowable material into a space formed between said innermould and said outer mould such that the flowable material forms ahelical element which will form at least part of an inner surface ofsaid drum;

d) removing the at least one outer mould;

e) removing the helical element from said inner mould;

f) repeating steps a)-e) so as to form a second helical element;

g) preparing an exterior surface of the helical elements for bonding toa structural layer of glass fibre.

Preferably, the flowable material is polyurethane.

In another broad form of an apparatus aspect the present inventioncomprises

a vehicle mounted rotary concrete mixing drum of the type having anopening at one end for receiving and/or discharge of concrete therefromand at the other end, means for engaging a drive assembly so as torotate the drum for mixing or discharging concrete; wherein, the drum ismanufactured from at least one mould using at least one plasticsmaterial; wherein the drum further includes integrally attached vaneswhich outstand from the internal surface of the drum forming anarchimedian spiral disposed such that when the drum is rotated in afirst direction the concrete contents are mixed and when the drum isrotated in a second direction the contents are discharged from saiddrum; wherein, the drum is formed by a method comprising the steps of;

a) preparing a first generally helical inner mold part containing asurface extending between first and second helical edges;

b) mounting the first helical inner mould part on a support

c) enclosing the inner helical mold assembly within an outer mouldformed by at least one outer mold part;

d) fitting a second mating inner helical mold part to the first innermould part to form an inner mold assembly;

e) injecting a polyurethane elastomer into a cavity defined by saidinner mold assembly and the outer mould assembly to form an inner wallelement comprising one half of an interior wall of the mixer and onehelical blade;

f) allowing said polyurethane to cure;

g) removing said at least one outer mold parts to expose said inner wallelement;

h) removing said inner wall element one of said inner molds;

In another broad form of the apparatus aspect, the present inventioncomprises:

a vehicle mounted concrete mixing drum formed by a method of manufacturecomprising the steps of;

a) taking a helical inner mould part and mounting the mould on asupport;

b) placing at least one external mould in opposing relationship to saidinner mould;

c) injecting a flowable material into a space formed between said innermould and said outer mould such that the flowable material forms ahelical element which will form at least part of an inner surface ofsaid drum;

d) removing the at least one outer mould;

e) removing the helical element from said inner mould;

f) repeating steps a)-e) so as to form a second helical element;

g) preparing an exterior surface of the helical elements for bonding toa structural layer of glass fibre.

Preferably the flowable material is polyurethane.

According to one embodiment the helical blades projecting from an innersurface of said drum have a pitch dimension of between 1-2 meters andare formed by elastomeric material. Preferably the wall strength of saiddrum is around 600 MPa at a wall thickness of 8 mm. Preferably thepolyurethane forms an inner layer which is approximately 3 mm thick.

Preferably the structural layer comprises filament windings forming alayer of approximately 5 mm thickness.

In another broad form the present invention comprises;

a heavy duty rotary concrete mixing drum capable of attachment to avehicle; the drum comprising a first end which engages a drive assemblywhich rotates said drum for mixing of said concrete and a second endfrom which mixed concrete is discharged; wherein said drum ismanufactured from at least one layer of plastics material wherein thedrum includes a wall having integral internal formations which promotemixing of said concrete and an inner surface which promotes mixing ofthe concrete at the boundary layer between the concrete and said innersurface; wherein the drum is formed according to the method steps of:

a) taking a helical inner mould and mounting the mould on a support;

b) placing at least one external mould in opposing relationship to saidinner mould;

c) injecting polyurethane into a space formed between said inner mouldand said outer mould;

d) removing the at least one outer mould;

e) removing a helical blade part from said inner mould;

f) repeating steps a)-e) so as to form a second helical blade part;

g) mating the first and second helical blade parts with an interiorshell;

g) preparing an exterior of the drum for bonding to a structural layerof glass fibre;

h) winding the structural layer about said exterior.

DETAILED DESCRIPTION

The present invention will now be described according to a preferred butnon limiting embodiment and with reference to the accompanyingillustrations wherein:

FIG. 1 shows a side elevation of an inner helical mould part;

FIG. 2 shows a side elevation of the mould part of FIG. 1 includingexploded external moulds;

FIG. 3 shows a side view of the inner mould part of FIGS. 1 and 2 fullyenclosed by external mould sections

FIG. 3 a shows an enlarged view of external moulds exploded from ahelical blade part;

FIG. 3 b shows an enlarged view of the assembly of FIG. 3 a with outermould parts moulds assembled.

FIG. 4 shows outer mold sections exploded from the inner mould assemblyupon completion of injection of an elastomer.

FIG. 5 shows a helical blade part produced by and removed from the innermould assembly of FIG. 4;

FIG. 6 shows a coupling of separate and complementary helical bladeparts formed by the arrangements of FIGS. 1-5 and which form an innerwall of the drum;

FIG. 7 shows a housing for assembly in which the inner wall of the drumis placed after moulding helical blade parts for preparation of theblade parts to receive an outer structural layer;

FIG. 8 shows an assembly including a mandrel for mounting said innerpayer for applying an outer structural layer of glass fibre;

FIG. 9 shows a completed drum with track ring fitted.;

The concentrated wear points in the prior art steel drums reduces theworking life of the drums necessitating costly repair or replacement.Steel drums are fabricated from rolled flat sheets which form cones anda cylinder which are then joined together by welding. Archimedianspirals are then welded to the inner surface of the drum resulting in ahigh specific gravity vessel whose self weight reduces the amount ofconcrete which can be carried by the vehicle to which it is attached.The steel drums suffer from a number of disadvantages includingsusceptibility to abrasion at the junctions of the cylindrical andconical sections and the tendency for unwanted concrete build up at thesharp corners and crevices formed by the mixing blades. In addition, thesmooth internal surface of the steel drum promotes sliding abrasion andinhibits mixing at the boundary layer due to the low coefficient offriction at the concrete/metal interface.

The method to be described below is an alternative to both steel drumsand the plastics drums formed by the methods of manufacture described inInternational Patent application No. PCT/AU00/01226 to Rodgers andKhouri. According to the method, a plastics heavy duty concrete mixingdrum is formed using both internal and external moulds. The drumincludes an internal archimedian spiral formed by helical blades orvanes which mix concrete during rotation of the drum in one directionand discharge concrete when the drum is rotated in an oppositedirection. The drum is generally pear shaped and includes an opening atone end for entry and discharge of concrete.

The first step in the method according to one embodiment, involves theuse of an internal mould. A polyurethane drum interior complete withinternal helical blades is formed between an interior and an exteriormould set. The exterior moulds are easily removed after the polyurethaneis formed, however because the mixer is a closed vessel with thedischarge opening smaller than the maximum diameter, and due to thecomplexity of the blade moulds, it is not possible to form the interioras one complete piece and then remove the mould. Accordingly, theelastomeric drum interior is moulded in sections which can be removedfrom the moulds, and then joined to form the complete interior. Thisinterior is then reinforced with structural layers to complete themixer. The joining together of two complementary inner wall sectionswhich are each formed by the same method steps sequentially is a newapproach to forming a drum interior not known in the prior art.

Referring to FIG. 1 there is shown an assembly 10 which comprises asupport 12 which receives and retains thereon a helical inner mouldassembly 14. Mould assembly 14 is shown in profile by line [4] 14A.Mould assembly 14 is initially prepared in a separate mould having ahelical interior so that mould 3 14 conforms to the shape of thatinterior. Once mould assembly 14 is formed, it may be transferredmanually or otherwise for mounting on support 12 in preparation forreceiving external mould[s] parts 16, 18 (shown in FIG. 2). FIG. 2 showsa side elevation of the mould assembly 14 of FIG. 1 with explodedopposing external mould parts 16 and 18.

As shown by FIG. 3, mould parts 16 and 18 encapture mould assembly 14but leave a generally helical cavity therebetween. Once external mouldparts 16 and 18 have been secured and sealed, polyurethane is injectedinto the aforesaid cavity 19. It will be appreciated by persons skilledin the art that more than two external moulds 16, 18 may be used tofulfill the same objectives of encapsulating inner mould 3. FIG. 3 showsa side view of the inner mould 3 assembly 14 enclosed by external mouldsections or parts 16 and 18.

FIG. 3 a shows an enlarged view of inner mould[s] assembly parts 14A and14B exploded from inner mould assembly 14 and outer mould 16 revealing asection view of a part helical spiral or element 20 retrieved from thecavity 19 defined by inner mould[s] parts 14A and 14B and outer mould 16following injection of polyurethane.

FIG. 3 b shows an enlarged section view of the assembly of FIG. 3 a withmoulds 14 and 16 assembled. Mould parts 14A, 14B and mould 16 are shownin FIG. 3 b in abutting relationship with molded part, which is formedby injection of a polyurethane elastomer into a cavity 19 formed betweenmoulds 14, 16 and 18.

Referring to FIG. 4 there is shown outer mold sections 16 and 18exploded from the inner mould assembly 14 upon completion of injectionof an elastomer. Upon removal of inner mould sections or parts 14A, 14B,helical (or element) [8] 20 which has been formed from the elastomericmaterial injected into the cavity defined by assembled external mould[s]parts 16, 18 and mould 14 as shown in FIG. 3 may be released. FIG. 5shows a first helical element 20 formed in a first helical (archimedian)spiral. The above described process is repeated once again to form asecond helical (archimedian) helical element 24. Helical element 22forms one half of a completed shell having blades 28 which areintegrally disposed on an inner surface 32 of the drum 72 formed by theprocesses described herein. A repeat of steps described with referenceto FIGS. 1-5 result in a second helical inner element [9] 24 which cooperates with and is complementary to helical element [8] 22 to form acompleted inner wall 26 including integral helical blade 28. FIG. 6shows a pair of helical sections 22 and [9] 24 formed by the assemblyand process as described with reference to FIGS. 1-5. Sections 22 and[9] 24 are in the form of two parallel helical spirals spaced at 180degree axial phase difference each with a reinforcing rod 30 (see FIGS.3 a and 3 b) in the interior edge, which may be a continuous filamentand resin rope. As these blades 28 are integral with the interiorsurface 32 of the mixer, it is convenient to join these two structuresalong two helical lines 34 mid way between the blades 28. In this waythe interior is formed as two identical mouldings or sections 22, 24which can be removed from the interior mould 14 and exterior mouldsections 16, 18 and which also contain the complete reinforcing rods 30.

As shown by FIGS. 3 a and 3 b, the moulds 16, 18 contain a layer 38 ofglass reinforced plastic which bonds to the polyurethane of elements 22,24 which is are formed, against it. In this way the two mouldings orsections 20, 24 are formed with two helical jointing lines 34 mid waybetween the blades 28 and a stiff inner shell 40 ready to receive thestructural layers 42 is formed.

According to one embodiment, a preferred method of manufacturing themixer is as follows.

As shown by FIGS. 3 a and 3 b, inner helical mould 14B includes surfaces42, 44 which extend[s] from a joint line 46 mid way between two formedhelical blade (28) to a mid-line mould joint line 48 at an inner edge ofthe blade 28. Surface 44 faces towards the drive end of the mixer andcontains a recess 50 for the reinforcing rod or filament rope 30. Thenext step is fitting reinforcing rod 30 with polyurethane spacers (notshown), which centralize the rod 30 in a recess 50 between inner mould14A, 14B.

A mating helical mould 14A is prepared, fitted and is joined withsealing gaskets (not shown) to mould 14 b along their inner edge 48. Thehelical mould 52 is formed by moulds 14A, 14B and mould 16, the surfacesof which are pre coated with a layer 58 of glass fibre reinforcedplastic, to provide an interior moulding 58 to the polyurethaneelastomer. Seals 60 are incorporated into a joint formed between themould 16 and moulds 14A, 14B. Following this the polyurethane elastomeris moulded into the cavity 62 formed between the moulds 14, 16. Theelastomer polymerizes to form section 20 which comprises half theinterior layer of the mixing drum 64. The polyurethane is allowed tocure whereupon moulds 14A, 14B and 16 are removed to expose an innershell and outer surface of the polyurethane section 20. This allowsremoval of the helical section 20, the interior polyurethane moulding.The above process is repeated to provide a second helical section 24,having blade 28 and interior wall moulding 26.

The two helical section 22 and [9] 24 are then assembled in a jig 62 asshown in FIG. 7 whereupon they are joined together using a polyurethaneelastomer compound. An inflation pressure is applied to the interior ofthe mixer to ensure it conforms to the interior of the jointing jig 62.The top half of the jig 62 is removed and the external surface of thepolyethylene joints is covered with glass fibre reinforced plastics.Following that, the external surface of the moulded polyurethaneinterior of the mixer is prepared for bonding to the structural layer.At this stage of the processing a drive ring is applied to thepolyurethane layer which will form the inner layer or wall 26 of themixing drum. The jig 62 is removed and the now formed inner layer 26 ofthe drum is transferred to a winding station 64 as shown in FIG. 8.

A steel mandrel 66 is inserted into the open (discharge ) end 68 of themixer so that it reaches the drive ring (not shown) which is common toheavy duty drums of this type. The drive ring which imparts rotation tothe mixer is spigotted and drilled to suit a gear box flange (notshown). The glass fibre reinforced plastic exterior of the polyurethaneinterior 26 is bonded to the drive ring and allowed to cure. The glassfibre reinforced plastic exterior 70 is extended over the discharge endsupport flange which then forms an enclosed vessel against the mandrel.

Structural layers of glass fibre reinforced plasticare applied by one ofthe known methods standard to the industry such as:

a) Contact moulding of random and/or directional glass fibres and resin.

b) Filament winding.

c) Surface finishing by filling and grinding or/ Moulding a gel coatfinish using vacuum or pressure.

The winding step is conducted by winding device 64 after drum innerlayer is mounted on mandrel 66. (see FIG. 8). As represented by FIG. 9,the final step in the construction of the drum involves the installationof the track ring 80.

These are known in conventional heavy duty drums and comprise acylindrical rail attached to the mixer adjacent to the discharge end andwhich is supported by two rollers to allow the mixer to be rotated bythe gearbox at the drive end. The ring is held in position with rubbergaskets (not shown) which seal to the mixer and this space between thering and the mixer is filled with a liquid polyurethane which bonds toboth as it gels and cures.

In operation this elastomer transmits the loads from the mixer shell tothe steel track ring and hence to the steel support rollers. In this waythe concentrated loads are spread and only low stresses are transmittedvia the elastomer to the mixer shell.

According to one embodiment, a computer may be employed to program andcontrol the delivery of the polymer to the mold surface and theapplication of the structural layer.

The winding of a fibre reinforced structural layer may involve computercontrolled unwinding of resin wetted fibre rovings from around arotating former. The tensile strength of the windings may be in theorder of 600 MPa. To obtain the optimum physical properties of thefilament wound structure the fibres are aligned to the loads imposed inuse of the finished drum. Typical loadings on the drum are axial bendingunder weight of wet concrete, an applied dynamic load at the drive endof the drum, driving torque and support loads at discharge end trunionrolls. The winding pattern of the filaments 70 aligns the fibres towithstand bending stresses, increasing in angle and in wall thicknesstowards the discharge end to accommodate applied roller loads.

The rovings 70 which line the drum may alternatively be drawn throughthe resin bath and applied to the surface of the drum as a wide ribboncomprising thousands of tensioned fibres. The windings 70 overlap untilthe required thickness is reached. The surface of the drum may becovered with wet resin and small irregularities which need to beaddressed to provide the external finish. As a result of thisconstruction, the spiral mixing blades 28 inside the drum are capable ofwithstanding high bending and shear resistance during mixing operations.The inner elastomeric surface 32 is highly resistant to abrasion byconcrete yet it is softer and lighter than the steel equivalent. Thehigher resistance to abrasion is facilitated by the natural elasticdeformation of the elastomer which absorbs the kinetic energy of theconcrete particles without gouging of the surface material. In addition,due to the property of the inner surface which will preferably bepolyurethane, the concrete will be mixed rather than slide at theboundary layer ensuring efficient mixing of the concrete throughout themix and reduction of abrasion due to the smooth curves throughout theinterior of the drum.

Furthermore, the blades 28 are strengthened by their molding integrallywith the wall 26 of the drum and have a stiffness factor which willsustain all applied normal operating loads.

A further advantage in the use of plastics for the mixing drums lies inthe thermal properties of the plastics material. Hot conditions areundesirable for concrete mixing as they accelerate hydration reducingconcrete workability which is an essential property required immediatelyfollowing a concrete pour. In very hot climates, the conventional steelvehicle mounted mixing drums conduct high heat loads which increase heatat the concrete boundary layer due to contact with the super heated drumwall causing unwanted accelerated hydration. This phenomenon isdifficult to avoid with steel drums as the conductivity of steel leadsto high conductive heat transfer from the outer skin of the drum to theinner wall which is normally in contact with the concrete. In some hotclimates, ice is placed in the steel drums in an attempt to arresttemperature increase inside the drum. As concrete hydration is anexothermic reaction, it is sensitive to external temperatures.Accordingly it is desirable that the concrete temperature remainsacceptably low to ensure a satisfactory level of workability and toretard hydration. Steel drums heat up significantly and conduct heatthrough their thickness making the concrete vulnerable to the vagariesof temperature variation. Overheating of the concrete mix is a problemto be avoided and has in accordance with one aspect provided a method ofmanufacture of a plastics drum to take the place of the conventionalsteel drums thereby reducing the unwanted effects of high thermalconductivity typical of the steel drums. The plastics drum allows theconcrete to remain workable inside the drum for longer periods comparedto concrete in steel mixing drums under the same external temperatureconditions and transporting concrete.

The method of construction of a plastics concrete mixing drum asdescribed herein provides an alternative yet efficient method ofproduction of plastics drums. The methods described herein allow formass production with reduction in the number of production stepscompared to the known methods.

It will be recognized by persons skilled in the art that numerousvariations and modifications may be made to the invention as broadlydescribed herein without departing from the overall spirit and scope ofthe invention.

1. A method of manufacture of a vehicle mounted rotary concrete mixingdrum of the type having an opening at one end for receiving and/ordischarge of concrete therefrom and at the other end, means for engaginga drive assembly so as to rotate the drum for mixing or dischargingconcrete; wherein, the drum is manufactured from at least one mouldusing at least one plastics material; wherein the drum further includesintegrally attached vanes which outstand from the internal surface ofthe drum forming an archimedian spiral disposed such that when the drumis rotated in a first direction the concrete contents are mixed and whenthe drum is rotated in a second direction the contents are dischargedfrom said drum; wherein, the method comprises the steps of; a) preparinga first generally helical inner mold part; b) mounting the first helicalinner mould part on a support c) enclosing the inner helical moldassembly within an outer mould formed by at least one outer mold part;d) fitting a second mating inner helical mold part to the first innermould part to form an inner mold assembly; e) injecting a polyurethaneelastomer into a cavity defined by said inner mold assembly and theouter mould assembly to form an inner wall element comprising one halfof an interior wall of the mixer and one helical blade; f) allowing saidpolyurethane to cure; g) removing said at least one outer mold parts toexpose said inner wall element; h) removing said inner wall element oneof said inner molds;
 2. A method according to claim 1 comprising theadditional step of placing a reinforcing member in a recess formed insaid inner mold part.
 3. A method according to claim 2, wherein stepsa)-h) are repeated thereby providing a second helical inner wallelement.
 4. A method according to claim 3 wherein the first and secondhelical inner wall elements are complimentary and combine to form aninner wall surface of the mixing drum.
 5. A method according to claim 4comprising the further step of placing said first and second helicalinner wall elements into a jig where opposing edges of said elements areheld adjacent; the wall elements defining an inner cavity of said drum.6. A method according to claim 5 wherein said opposing edges are sealedto complete said inner wall of said drum.
 7. A method according to claim6 comprising the further step of removing said inner wall from said jigand placing said inner wall on a mandrel such that the mandrel isdisposed in said inner cavity.
 8. A method according to claim 7 whereinthe inner wall is placed on said mandrel via an open end of said innerwall
 9. A method according to claim 8 comprising the further step ofapplying structural layers of glass fibre reinforced plastic to thepolyurethane inner layer.
 10. A method according to claim 9 wherein theinternal surface of the drum includes an elastomer which allows mixingof the contents of the concrete at a concrete boundary layer;
 11. Amethod according to claim 10 wherein said reinforcing member is acontinuous rope inserted in a recess in said blades.
 12. A method ofconstruction of a plastics concrete mixing drum wherein the methodincludes the use of inner and outer molds each made up from separatemould parts which are divided along two helical lines thereby allowingformation of a drum interior wall from two complementary mould parts.13. A method of manufacture of a vehicle mounted rotary concrete mixingdrum of the type having an opening at one end for receiving anddischarge of concrete therefrom and at the other end means for engaginga drive assembly so as to rotate the drum for mixing or dischargingconcrete; wherein, the drum is manufactured from at least one innermould and at least one opposing outer mould; wherein the drum includesintegrally attached vanes which outstand from the internal surface ofthe drum forming an archimedian spiral disposed such that when the drumis rotated in a first direction the concrete contents are mixed and whenthe drum is rotated in a second direction the contents are dischargesfrom said drum; and wherein the internal surface of the drum is formedor lined with an elastomer which causes mixing of the contents of theconcrete at the concrete boundary layer; wherein the method comprisesthe steps of; a) preparing a first inner helical mold containing asurface intermediate side edges of the mould; b) placing a reinforcingrod in a recess in said inner mold; c) enclosing the inner helical moldassembly within at least one outer mold part; d) sealing a joint betweensaid inner mold part and said at least one outer mold part; e) injectinga polyurethane elastomer into a cavity defined by said inner mold andsaid at least one outer mold part to form an inner helical wall elementcomprising one half of an interior of the mixer and one helical blade;g) allowing said polyurethane to cure; h) removing said at least oneouter mold parts; i) removing one of said inner molds; j) removing theinterior polyurethane inner helical wall element molding from theremainder of the inner mold assembly; k) repeating steps a)-j) to form asecond inner wall element.
 14. A method according to claim 13 comprisingthe further steps of a) placing a second inner wall element along withsaid first inner helical wall element in a jig where the jointingsurfaces are held adjacent so as to form an inner wall; b) sealing thejoin formed by said jointing surfaces.
 15. A method according to claim14 wherein comprising the additional step of a) inserting a mandrel intoan open discharge end of a drum interior; b) winding structural layersof glass fibre reinforced plastic about an outer surface of said innerwall.
 16. A method of construction of a plastics mixing drum comprisingthe steps of: a) preparing a first inner mold containing a surfaceextending from a joint line midway between two helical blades to a midline mould joint line at an inner edge of said blades; b) placing areinforcing rod in a recess in said inner mold; c) fitting a secondmating inner helical mold to form an inner mold assembly; d) enclosingthe inner helical mold assembly within at least one outer mold part; e)sealing a joint between said inner mold assembly and said at least oneouter mold part; injecting a polyurethane elastomer into a cavitydefined by said inner mold assembly and said at least one outer moldpart to form one half of the interior of the mixer and one of thehelical blades; f) allowing said polyurethane to cure; g) removing saidat least one outer mold parts; h) removing one of said inner molds; i)removing the interior polyurethane molding from the remainder of theinner mold assembly; j) placing said two helical blade and interiormoldings in a jig where the jointing surfaces are held adjacent; k)inserting a mandrel into an open discharge end of the drum; l) applyingstructural layers of glass fibre reinforced plastic the polyurethanelayer.
 17. A method according to claim 16 wherein the reinforcing rod isfitted with spacers which centralize the rod in its recess.
 18. A methodaccording to claim 17 wherein, the first and second inner helical moldelements are jointed with a sealing compound or gaskets along an inneredge.
 19. A method according to claim 17 wherein the outer mold isformed in three separate mold parts.
 20. A method according to claim 19wherein, the joint between said two inner helical elements forming ahelical blade is made with a polyurethane elastomer compound.
 21. Amethod of manufacture of a vehicle mounted concrete mixing drumcomprising the steps of; a) taking a helical inner mould part andmounting the mould on a support; b) placing at least one external mouldin opposing relationship to said inner mould; c) injecting a flowablematerial into a space formed between said inner mould and said outermould such that the flowable material forms a helical element which willform at least part of an inner surface of said drum; d) removing the atleast one outer mould; e) removing the helical element from said innermould; f) repeating steps a)-e) so as to form a second helical element;g) preparing an exterior surface of the helical elements for bonding toa structural layer of glass fibre.
 22. A method according to claim 21wherein the flowable material is polyurethane.
 23. A method according toclaim 22 wherein, said first and second helical elements are joined in ajig to form an inner surface of said drum.
 24. A method according toclaim 23 comprising the further step of preparing an exterior surface ofthe mixer for bonding to a structural layer of glass fibre.
 25. A methodaccording to claim 24 comprising the additional step of winding astructural layer of fiberglass about said exterior surface.
 26. Avehicle mounted rotary concrete mixing drum of the type having anopening at one end for receiving and/or discharge of concrete therefromand at the other end, means for engaging a drive assembly so as torotate the drum for mixing or discharging concrete; wherein, the drum ismanufactured from at least one mould using at least one plasticsmaterial; wherein the drum further includes integrally attached vaneswhich outstand from the internal surface of the drum forming anarchimedian spiral disposed such that when the drum is rotated in afirst direction the concrete contents are mixed and when the drum isrotated in a second direction the contents are discharged from saiddrum; wherein, the drum is formed by a method comprising the steps of;a) preparing a first generally helical inner mold part containing asurface extending between first and second helical edges; b) mountingthe first helical inner mould part on a support c) enclosing the innerhelical mold assembly within an outer mould formed by at least one outermold part; d) fitting a second mating inner helical mold part to thefirst inner mould part to form an inner mold assembly; e) injecting apolyurethane elastomer into a cavity defined by said inner mold assemblyand the outer mould assembly to form an inner wall element comprisingone half of an interior wall of the mixer and one helical blade; f)allowing said polyurethane to cure; g) removing said at least one outermold parts to expose said inner wall element; h) removing said innerwall element one of said inner molds;
 27. A vehicle mounted concretemixing drum formed by a method of manufacture comprising the steps of;a) taking a helical inner mould part and mounting the mould on asupport; b) placing at least one external mould in opposing relationshipto said inner mould; c) injecting a flowable material into a spaceformed between said inner mould and said outer mould such that theflowable material forms a helical element which will form at least partof an inner surface of said drum; d) removing the at least one outermould; e) removing the helical element from said inner mould; f)repeating steps a)-e) so as to form a second helical element; g)preparing an exterior surface of the helical elements for bonding to astructural layer of glass fibre.
 28. A concrete mixing drum according toclaim 27 wherein the flowable material is polyurethane.
 29. A concretemixing drum according to claim 27 wherein helical blades projecting froman inner surface of said drum have a pitch dimension of between 1-2meters and are formed by elastomeric material.
 30. A mixing drumaccording to claim 28 wherein the wall strength of said drum is around600 MPa at a wall thickness of 8mm.
 31. A mixing drum according to claim29 wherein the polyurethane forms an inner layer which is approximately3 mm thick.
 32. A mixing drum according to claim 31 wherein and saidstructural layer comprises filament windings forming a layer ofapproximately 5mm thickness.
 33. A method for making a concrete mixingdrum, the method comprising: combining a first helical section and asecond helical section.
 34. A method for making a concrete mixing drum,the method comprising: providing a mould having a recess configured toform a drum blade; locating a reinforcing member in the recess; andinserting polymeric fluid in the recess such that the fluid flows aboutand substantially surrounds the reinforcing member.
 35. A concretemixing drum comprising: a first helical element; and a second helicalelement joined to the first helical element, wherein the first helicalelement and the second helical element form a substantially continuouslayer circumferentially extending about a longitudinal axis of the drum.36. The drum of claim 35 wherein the first helical element and thesecond helical element are polymeric.
 37. The drum of claim 35 whereinthe first helical element includes a blade.
 38. The drum of claim 37wherein the second helical element includes a blade.
 39. The drum ofclaim 37 wherein the blade is helical.
 40. The drum of claim 37including a reinforcing member within a tip of the blade.
 41. The drumof claim 35 including a second substantially continuous layer extendingacross a junction of the first element and the second element.
 42. Thedrum of claim 41 wherein the second continuous layer is a fibrereinforced elastomer.
 43. The drum of claim 35 wherein the first helicalelement and the second helical element form an interior of the drum. 44.An element comprising: a helical polymeric shell; and a polymeric bladeintegrally formed with and extending from the shell.